Transversely-excited film bulk acoustic resonator matrix filters with split die sub-filters

ABSTRACT

A radio frequency filter includes at least a first sub-filter and a second sub-filter connected in parallel between a first port and a second port. Each of the sub-filters has a piezoelectric plate having front and back surfaces, the back surface attached to a substrate, and portions of the piezoelectric plate forming diaphragms spanning respective cavities in the substrate. A conductor pattern is formed on the front surface of the plate, the conductor pattern includes interdigital transducers (IDTs) of a respective plurality of resonators, with interleaved fingers of each IDT disposed on a respective diaphragm of the plurality of diaphragms. A thickness of the portions of the piezoelectric plate of the first sub-filter is different from a thickness of the portions of the piezoelectric plate of the second sub-filter.

RELATED APPLICATION INFORMATION

This patent is a continuation of co-pending U.S. patent application Ser.No. 17/362,727, filed Jun. 29, 2021, titled TRANSVERSELY-EXCITED FILMBULK ACOUSTIC RESONATOR MATRIX FILTERS WITH SPLIT DIE SUB-FILTERS, whichclaims priority to provisional patent application 63/127,095, filed Dec.17, 2020, titled SPLIT SUB-FILTER MATRIX XBAR FILTER and is acontinuation-in-part of U.S. patent application Ser. No. 17/133,849,filed Dec. 24, 2020, titled TRANSVERSELY-EXCITED FILM BULK ACOUSTICRESONATOR MATRIX FILTERS, which is a continuation-in-part of applicationSer. No. 17/121,724, filed Dec. 14, 2020, titled ACOUSTIC MATRIX FILTERSAND RADIOS USING ACOUSTIC MATRIX FILTERS, which claims priority fromprovisional patent application 63/087,789, filed Oct. 5, 2020, entitledMATRIX XBAR FILTER. All of these applications are incorporated herein byreference.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. This patent document may showand/or describe matter which is or may become trade dress of the owner.The copyright and trade dress owner has no objection to the facsimilereproduction by anyone of the patent disclosure as it appears in thePatent and Trademark Office patent files or records, but otherwisereserves all copyright and trade dress rights whatsoever.

BACKGROUND Field

This disclosure relates to radio frequency filters using acoustic waveresonators, and specifically to filters for use in communicationsequipment.

Description of the Related Art

A radio frequency (RF) filter is a two-port device configured to passsome frequencies and to stop other frequencies, where “pass” meanstransmit with relatively low signal loss and “stop” means block orsubstantially attenuate. The range of frequencies passed by a filter isreferred to as the “pass-band” of the filter. The range of frequenciesstopped by such a filter is referred to as the “stop-band” of thefilter. A typical RF filter has at least one pass-band and at least onestop-band. Specific requirements on a passband or stop-band depend onthe specific application. For example, a “pass-band” may be defined as afrequency range where the insertion loss of a filter is better than adefined value such as 1 dB, 2 dB, or 3 dB. A “stop-band” may be definedas a frequency range where the rejection of a filter is greater than adefined value such as 20 dB, 30 dB, 40 dB, or greater depending onapplication.

RF filters are used in communications systems where information istransmitted over wireless links. For example, RF filters may be found inthe RF front-ends of cellular base stations, mobile telephone andcomputing devices, satellite transceivers and ground stations, IoT(Internet of Things) devices, laptop computers and tablets, fixed pointradio links, and other communications systems. RF filters are also usedin radar and electronic and information warfare systems.

RF filters typically require many design trade-offs to achieve, for eachspecific application, the best compromise between performance parameterssuch as insertion loss, rejection, isolation, power handling, linearity,size and cost. Specific design and manufacturing methods andenhancements can benefit simultaneously one or several of theserequirements.

Performance enhancements to the RF filters in a wireless system can havebroad impact to system performance. Improvements in RF filters can beleveraged to provide system performance improvements such as larger cellsize, longer battery life, higher data rates, greater network capacity,lower cost, enhanced security, higher reliability, etc. Theseimprovements can be realized at many levels of the wireless system bothseparately and in combination, for example at the RF module, RFtransceiver, mobile or fixed sub-system, or network levels.

High performance RF filters for present communication systems commonlyincorporate acoustic wave resonators including surface acoustic wave(SAW) resonators, bulk acoustic wave (BAW) resonators, film bulkacoustic wave resonators (FBAR), and other types of acoustic resonators.However, these existing technologies are not well-suited for use at thehigher frequencies and bandwidths proposed for future communicationsnetworks.

The desire for wider communication channel bandwidths will inevitablylead to the use of higher frequency communications bands. Radio accesstechnology for mobile telephone networks has been standardized by the3GPP (3^(rd) Generation Partnership Project). Radio access technologyfor 5^(th) generation mobile networks is defined in the 5G NR (newradio) standard. The 5G NR standard defines several new communicationsbands. Two of these new communications bands are n77, which uses thefrequency range from 3300 MHz to 4200 MHz, and n79, which uses thefrequency range from 4400 MHz to 5000 MHz. Both band n77 and band n79use time-division duplexing (TDD), such that a communications deviceoperating in band n77 and/or band n79 use the same frequencies for bothuplink and downlink transmissions. Bandpass filters for bands n77 andn79 must be capable of handling the transmit power of the communicationsdevice. WiFi bands at 5 GHz and 6 GHz also require high frequency andwide bandwidth. The 5G NR standard also defines millimeter wavecommunication bands with frequencies between 24.25 GHz and 40 GHz.

The Transversely-Excited Film Bulk Acoustic Resonator (XBAR) is anacoustic resonator structure for use in microwave filters. The XBAR isdescribed in U.S. Pat. No. 10,491,291, titled TRANSVERSELY EXCITED FILMBULK ACOUSTIC RESONATOR, which is incorporated herein by reference. AnXBAR resonator comprises an interdigital transducer (IDT) formed on athin floating layer, or diaphragm, of a single-crystal piezoelectricmaterial. The IDT includes a first set of parallel fingers, extendingfrom a first busbar and a second set of parallel fingers extending froma second busbar. The first and second sets of parallel fingers areinterleaved. A microwave signal applied to the IDT excites a shearprimary acoustic wave in the piezoelectric diaphragm. XBAR resonatorsprovide very high electromechanical coupling and high frequencycapability. XBAR resonators may be used in a variety of RF filtersincluding band-reject filters, band-pass filters, duplexers, andmultiplexers. XBARs are well suited for use in filters forcommunications bands with frequencies above 3 GHz. Matrix XBAR filtersare also suited for frequencies between 1 GHz and 3 GHz.

DESCRIPTION OF THE DRAWINGS

FIG. 1 includes a schematic plan view, two schematic cross-sectionalviews, and a detailed cross-sectional view of a transversely-excitedfilm bulk acoustic resonator (XBAR).

FIG. 2A is an equivalent circuit model of an acoustic resonator.

FIG. 2B is a graph of the admittance of an ideal acoustic resonator.

FIG. 2C is a circuit symbol for an acoustic resonator.

FIG. 3A is a schematic diagram of a matrix filter using acousticresonators.

FIG. 3B is a schematic diagram of a sub-filter of FIG. 3A.

FIG. 4 is a schematic diagram of a matrix filter usingtransversely-excited film bulk acoustic resonators.

FIG. 5 is plain view of an embodiment of the matrix filter of FIG. 4 .

FIG. 6 is a schematic cross-sectional view of the matrix filter of FIG.5 .

FIG. 7 is a graph of input-output transfer functions of an embodiment ofthe matrix filter of FIGS. 4-6 .

FIG. 8 is flow chart of a process for making a matrix filter usingtransversely-excited film bulk acoustic resonators.

Throughout this description, elements appearing in figures are assignedthree-digit or four-digit reference designators, where the two leastsignificant digits are specific to the element and the one or two mostsignificant digit is the figure number where the element is firstintroduced. An element that is not described in conjunction with afigure may be presumed to have the same characteristics and function asa previously-described element having the same reference designator.

DETAILED DESCRIPTION

Description of Apparatus

The Transversely-Excited Film Bulk Acoustic Resonator (XBAR) is a newresonator structure for use in acoustic filters for filtering microwavesignals. The XBAR is described in U.S. Pat. No. 10,491,291, titledTRANSVERSELY EXCITED FILM BULK ACOUSTIC RESONATOR, which is incorporatedherein by reference in its entirety. An XBAR resonator comprises aconductor pattern having an interdigital transducer (IDT) formed on athin floating layer or diaphragm of a piezoelectric material. The IDThas two busbars which are each attached to a set of fingers and the twosets of fingers are interleaved on the diaphragm over a cavity formed ina substrate upon which the resonator is mounted. The diaphragm spans thecavity and may include front-side and/or back-side dielectric layers. Amicrowave signal applied to the IDT excites a shear primary acousticwave in the piezoelectric diaphragm, such that the acoustic energy flowssubstantially normal to the surfaces of the layer, which is orthogonalor transverse to the direction of the electric field generated by theIDT. XBAR resonators provide very high electromechanical coupling andhigh frequency capability.

Acoustic filters are typically required to match a system impedance,such as 50 ohms. The system impedance and operating frequency dictate arequired equivalent capacitance C₀ for a filter using a conventionalladder circuit. C0 is inversely proportional to frequency. XBARresonators have low capacitance per unit area compared to other acousticresonators. Thus, ladder filter circuits using XBAR resonators may bemuch larger than comparable filters using other types of acousticresonators.

The following describes a filter circuit architecture that allows lowfrequency filters to be implemented with small XBAR resonators. It alsodescribes improved XBAR resonators, filters and fabrication techniquesthat reduce static capacitance in radio frequency filters havingsub-filters connected in parallel between two ports where thesub-filters have XBARs on different substrates of different die. Thesub-filter XBARs have a piezoelectric plate with a back surface attachedto the different substrates and portions of the plate forming diaphragmsspanning cavities in the substrates. Interleaved fingers of IDTs are onthe diaphragms and the thicknesses of the piezoelectric plate portionsmay be different.

FIG. 1 shows a simplified schematic top view, orthogonal cross-sectionalviews, and a detailed cross-sectional view of a transversely-excitedfilm bulk acoustic resonator (XBAR) 100. XBAR resonators such as theresonator 100 may be used in a variety of RF filters includingband-reject filters, band-pass filters, duplexers, and multiplexers.XBARs are particularly suited for use in filters for communicationsbands with frequencies above 3 GHz. The matrix XBAR filters described inthis patent are also suited for frequencies above 1 GHz.

The XBAR 100 is made up of a thin film conductor pattern formed on asurface of a piezoelectric plate 110 having parallel front and backsurfaces 112, 114, respectively. The piezoelectric plate is a thinsingle-crystal layer of a piezoelectric material such as lithiumniobate, lithium tantalate, lanthanum gallium silicate, gallium nitride,or aluminum nitride. The piezoelectric plate is cut such that theorientation of the X, Y, and Z crystalline axes with respect to thefront and back surfaces is known and consistent. The piezoelectric platemay be Z-cut (which is to say the Z axis is normal to the front and backsurfaces 112, 114), rotated Z-cut, or rotated YX cut. XBARs may befabricated on piezoelectric plates with other crystallographicorientations.

The back surface 114 of the piezoelectric plate 110 is attached to asurface of the substrate 120 except for a portion of the piezoelectricplate 110 that forms a diaphragm 115 spanning a cavity 140 formed in thesubstrate. The portion of the piezoelectric plate that spans the cavityis referred to herein as the “diaphragm” 115 due to its physicalresemblance to the diaphragm of a microphone. As shown in FIG. 1 , thediaphragm 115 is contiguous with the rest of the piezoelectric plate 110around all of a perimeter 145 of the cavity 140. In this context,“contiguous” means “continuously connected without any interveningitem”. In other configurations, the diaphragm 115 may be contiguous withthe piezoelectric plate around at least 50% of the perimeter 145 of thecavity 140.

The substrate 120 provides mechanical support to the piezoelectric plate110. The substrate 120 may be, for example, silicon, sapphire, quartz,or some other material or combination of materials. The back surface 114of the piezoelectric plate 110 may be bonded to the substrate 120 usinga wafer bonding process. Alternatively, the piezoelectric plate 110 maybe grown on the substrate 120 or attached to the substrate in some othermanner. The piezoelectric plate 110 may be attached directly to thesubstrate or may be attached to the substrate 120 via one or moreintermediate material layers (not shown in FIG. 1 ).

“Cavity” has its conventional meaning of “an empty space within a solidbody.” The cavity 140 may be a hole completely through the substrate 120(as shown in Section A-A and Section B-B) or a recess in the substrate120 under the diaphragm 115. The cavity 140 may be formed, for example,by selective etching of the substrate 120 before or after thepiezoelectric plate 110 and the substrate 120 are attached.

The conductor pattern of the XBAR 100 includes an interdigitaltransducer (IDT) 130. The IDT 130 includes a first plurality of parallelfingers, such as finger 136, extending from a first busbar 132 and asecond plurality of fingers extending from a second busbar 134. Thefirst and second pluralities of parallel fingers are interleaved. Theinterleaved fingers overlap for a distance AP, commonly referred to asthe “aperture” of the IDT. The center-to-center distance L between theoutermost fingers of the IDT 130 is the “length” of the IDT.

The first and second busbars 132, 134 serve as the terminals of the XBAR100. A radio frequency or microwave signal applied between the twobusbars 132, 134 of the IDT 130 excites a primary acoustic mode withinthe piezoelectric plate 110. The primary acoustic mode of an XBAR is abulk shear mode where acoustic energy propagates along a directionsubstantially orthogonal to the surface of the piezoelectric plate 110,which is also normal, or transverse, to the direction of the electricfield created by the IDT fingers. Thus, the XBAR is considered atransversely-excited film bulk wave resonator.

The IDT 130 is positioned on the piezoelectric plate 110 such that atleast the fingers of the IDT 130 are disposed on the diaphragm 115 ofthe piezoelectric plate which spans, or is suspended over, the cavity140. As shown in FIG. 1 , the cavity 140 has a rectangular shape with anextent greater than the aperture AP and length L of the IDT 130. Acavity of an XBAR may have a different shape, such as a regular orirregular polygon. The cavity of an XBAR may have more or fewer thanfour sides, which may be straight or curved.

The detailed cross-section view (Detail C) shows two IDT fingers 136 a,136 b on the surface of the piezoelectric plate 110. The dimension p isthe “pitch” of the IDT and the dimension w is the width or “mark” of theIDT fingers. A dielectric layer 150 may be formed between and optionallyover (see IDT finger 136 a) the IDT fingers. The dielectric layer 150may be a non-piezoelectric dielectric material, such as silicon dioxideor silicon nitride. The dielectric layer 150 may be formed of multiplelayers of two or more materials. The IDT fingers 136 a and 136 b may bealuminum, copper, beryllium, gold, tungsten, molybdenum, alloys andcombinations thereof, or some other conductive material. Thin (relativeto the total thickness of the conductors) layers of other metals, suchas chromium or titanium, may be formed under and/or over and/or aslayers within the fingers to improve adhesion between the fingers andthe piezoelectric plate 110 and/or to passivate or encapsulate thefingers and/or to improve power handling. The busbars of the IDT 130 maybe made of the same or different materials as the fingers.

For ease of presentation in FIG. 1 , the geometric pitch and width ofthe IDT fingers is greatly exaggerated with respect to the length(dimension L) and aperture (dimension AP) of the XBAR. A typical XBARhas more than ten parallel fingers in the IDT 110. An XBAR may havehundreds of parallel fingers in the IDT 110. Similarly, the thickness ofthe fingers in the cross-sectional views is greatly exaggerated.

An XBAR based on shear acoustic wave resonances can achieve betterperformance than current state-of-the art surface acoustic wave (SAW),film-bulk-acoustic-resonators (FBAR), and solidly-mounted-resonatorbulk-acoustic-wave (SMR BAW) devices. In particular, the piezoelectriccoupling for shear wave XBAR resonances can be high (>20%) compared toother acoustic resonators. High piezoelectric coupling enables thedesign and implementation of microwave and millimeter-wave filters ofvarious types with appreciable bandwidth.

The basic behavior of acoustic resonators, including XBARs, is commonlydescribed using the Butterworth Van Dyke (BVD) circuit model as shown inFIG. 2A. The BVD circuit model consists of a motional arm and a staticarm. The motional arm includes a motional inductance L_(m), a motionalcapacitance C_(m), and a resistance R_(m). The static arm includes astatic capacitance C₀ and a resistance R₀. While the BVD model does notfully describe the behavior of an acoustic resonator, it does a good jobof modeling the two primary resonances that are used to design band-passfilters, duplexers, and multiplexers (multiplexers are filters with morethan 2 input or output ports with multiple passbands).

The first primary resonance of the BVD model is the motional resonancecaused by the series combination of the motional inductance L_(m) andthe motional capacitance C_(m). The second primary resonance of the BVDmodel is the anti-resonance caused by the combination of the motionalinductance L_(m), the motional capacitance C_(m), and the staticcapacitance C₀. In a lossless resonator (R_(m)=R₀=0), the frequencyF_(r) of the motional resonance is given by

$\begin{matrix}{F_{r} = \frac{1}{2\pi\sqrt{L_{m}C_{m}}}} & (1)\end{matrix}$The frequency F_(a) of the anti-resonance is given by

$\begin{matrix}{F_{a} = {F_{r}\sqrt{1 + \frac{1}{\gamma}}}} & (2)\end{matrix}$where γ=C₀/C_(m) is dependent on the resonator structure and the typeand the orientation of the crystalline axes of the piezoelectricmaterial.

FIG. 2B is a graph 200 of the magnitude of admittance of a theoreticallossless acoustic resonator. The acoustic resonator has a resonance 212at a resonance frequency where the admittance of the resonatorapproaches infinity. The resonance is due to the series combination ofthe motional inductance L_(m) and the motional capacitance C_(m) in theBVD model of FIG. 2A. The acoustic resonator also exhibits ananti-resonance 214 where the admittance of the resonator approacheszero. The anti-resonance is caused by the combination of the motionalinductance L_(m), the motional capacitance C_(m), and the staticcapacitance C₀. In a lossless resonator (R_(m)=R₀=0), the frequencyF_(r) of the resonance is given by

$\begin{matrix}{F_{r} = \frac{1}{2\pi\sqrt{L_{m}C_{m}}}} & (1)\end{matrix}$The frequency F_(a) of the anti-resonance is given by

$\begin{matrix}{F_{a} = {F_{r}\sqrt{1 + \frac{1}{\gamma}}}} & (2)\end{matrix}$

In over-simplified terms, the lossless acoustic resonator can beconsidered a short circuit at the resonance frequency 212 and an opencircuit at the anti-resonance frequency 214. The resonance andanti-resonance frequencies in FIG. 2B are representative, and anacoustic resonator may be designed for other frequencies.

FIG. 2C shows the circuit symbol for an acoustic resonator such as anXBAR. This symbol will be used to designate each acoustic resonator inschematic diagrams of filters in subsequent figures.

FIG. 3A is a schematic diagram of a matrix filter 300 using acousticresonators. The matrix filter 300 includes an array 310 of n sub-filters320-1, 320-2, 320-n connected in parallel between a first filter port(FP1) and a second filter port (FP2), where n is an integer greater thanone. Each of the n sub-filters 320-1, 320-2, 320-n is a bandpass filterhaving a bandwidth about 1/n times the bandwidth of the matrix filter300. The sub-filters 320-1, 320-2, 320-n have contiguous passbands suchthat the bandwidth of the matrix filter 300 is equal to the sum of thebandwidths of the constituent sub-filters. In the subsequent examples inthis patent n=3. n can be less than or greater than 3 as necessary toprovide the desired bandwidth for the matrix filter 300. In some cases,the n sub-filters 320-1, 320-2, 320-n may include one or more XBARs. Thefilter 300 and/or sub-filters may be RF filters that pass frequencybands defined by the 5G NR standard.

The array 310 of sub-filters is terminated at the FP1 end by acousticresonators XL1 and XH1, which are preferably but not necessarily XBARs.The array 310 of sub-filters is terminated at the FP2 end by acousticresonators XL2 and XH2, which are preferably but not necessarily XBARs.The acoustic resonators XL1, XL2, XH1, and XH2 create “transmissionzeros” at their respective resonance frequencies. A “transmission zero”is a frequency where the input-output transfer function of the filter300 is very low (and would be zero if the acoustic resonators XL1, XL2,XH1, and XH2 were lossless). The zero transmission may be caused by oneor more of the acoustic resonators creating a very low impedance toground and thus, in this configuration cause the sub-filters to beremoved as filtering components as the acoustic resonators are basicallyshort circuits to ground so that the sub-filters have no effect on thefilter 300 during transmission zero frequencies. Typically, but notnecessarily, the resonance frequencies of XL1 and XL2 are equal, and theresonance frequencies of XH1 and XH2 are equal. The resonant frequenciesof the acoustic resonators XL1, XL2 are selected to provide transmissionzeros adjacent to the lower edge of the filter passband. XL1 and XL2 maybe referred to as “low-edge resonators” since their resonant frequenciesare proximate the lower edge of the filter passband. The acousticresonators XL1 and XL2 also act as shunt inductances to help match theimpedance at the ports of the filter to a desired impedance value. Inthe subsequent examples in this patent, the impedance at all ports ofthe filters is matched to 50 ohms. The impedance may be another value ifdesired, such as 20, 100 or 1000 ohms. The resonant frequencies ofacoustic resonators XH1, XH2 are selected to provide transmission zerosat or above the higher edge of the filter passband. XH1 and XH2 may bereferred to as “high-edge resonators” since their resonant frequenciesare proximate the higher edge of the filter passband. High-edgeresonators XH1 and XH2 may not be required in all matrix filters, suchas filters where high rejection above the passband is not required.

FIG. 3B is a schematic diagram of a sub-filter 350 suitable for each ofsub-filters 320-1, 320-2, and 320-n of filter 300. The sub-filter 350includes three acoustic resonators XA, XB, XC connected in seriesbetween a first sub-filter port (SP1) which can be connected to FP1 anda second sub-filter port (SP2) which can be connected to FP2. Theacoustic resonators XA, XB, XC are preferably but not necessarily XBARs.The sub-filter 350 includes two coupling capacitors CA, CB, each ofwhich is connected between ground and a respective node between two ofthe acoustic resonators. The inclusion of three acoustic resonators inthe sub-filter 350 is exemplary. A sub-filter may have m acousticresonators, where m is an integer greater than one. A sub-filter with macoustic resonators includes m−1 coupling capacitors The m acousticresonators of a sub-filter are connected in series between the two portsSP1 and SP2 of a sub-filter and each of the m−1 coupling capacitors isconnected between ground and a node between a respective pair ofacoustic resonators from the in acoustic resonators.

Compared to other types of acoustic resonators, XBARs have very highelectromechanical coupling (which results in a large difference betweenthe resonance and anti-resonance frequencies), but low capacitance perunit area. The matrix filter architecture, as shown in FIG. 3A and FIG.3B, takes advantage of the high electromechanical coupling of XBARswithout requiring high resonator capacitance. Thus, this architectureimproves high frequency bandpass filtering by passing a wider range ofhigh frequency without requiring processing to form or the space to formhigh-edge resonators XH1 and XH2.

FIG. 4 is a schematic circuit diagram of an exemplary matrix filter 400implemented with XBARs. The matrix filter 400 includes three sub-filters420-1, 420-2, 420-3 connected in parallel between a first filter port(FP1) and a second filter port (FP2). The sub-filters 420-1, 420-2,420-3 have contiguous passbands such that the bandwidth of the matrixfilter 300 is equal to the sum of the bandwidths of the constituentsub-filters. Each sub-filter includes three XBARs connected in seriesand two coupling capacitors. For example, sub-filter 420-1 includesseries XBARs X1A, X1B, and X1C and two coupling capacitors C1A, C1B eachof which is connected between ground and a respective node between twoof the acoustic resonators. Components of the other sub-filters 420-2and 420-3 are similarly identified using 2's and 3's as those using 1'sin sub-filter 420-1. Low-edge XBARs XL1 and XL2 are connected betweenFP1 and FP2, respectively, and ground. All of the capacitors within thesub-filters are connected to ground through a common inductor L1. Theinclusion of the inductor L1 improves the out-of-band rejection of thematrix filter 400 which improves filtering. The matrix filter 400 doesnot include high-edge resonators.

The exemplary matrix filter 400 is symmetrical in that the impedances atFP1 and FP2 are both equal to 50 ohms. The impedance may be anothervalue if desired, such as 20, 100 or 1000 ohms. The internal circuitryof the filter is also symmetrical, with XBARs X_A and X_C within eachsub-filter being the same and low-edge resonators XL1 and XL2 being thesame. Other matrix filters may be designed to have significantlydifferent impedances at FP1 and FP2, in which event the internalcircuitry will not be symmetrical.

FIG. 5 is a plan view of an exemplary matrix filter 500 which has thesame schematic circuit diagram as the matrix filter 400 of FIG. 4 . Theexemplary matrix filter is an LTE band 41 bandpass filter with apassband from 2496 to 2690 MHz.

The matrix filter 500 includes three Z-cut lithium tantalatepiezoelectric plate thickness portions 510-3 for filter 410-3, 510-1 forfilter 410-1, and 510-2 for filter 410-2. The portions 510-1, 2 and 3may be three different piezoelectric diaphragm thicknesses. Any numberof portions 510-1, 2 and 3 can be on any number of plates; and anynumber of those plates can be on any number of substrates of differentdie or chips. Other matrix filters may use lithium niobate piezoelectricplates and other crystal orientations including rotated Z-cut androtated Y-cut.

For example, the portions 510-1, 2 and 3 could be three separate plateseach having a different thickness, or three portions of the same platehaving different thicknesses created using a process multiplethicknesses on the same plate. Such a process may thin portions 510-2and 3 from the thickness of portion 510-1; and then further thin portion510-3 from the thickness of portion 510-2.

The back surface of each plate is attached to one substrate. In anothercase it is attached to more than one substrate. The back surface of eachof portions 510-1, 2 and 3 is bonded to a substrate. Whether or not theyare of the same or of separate plates, in a first case, the portions510-1, 2 and 3 are each bonded to a separate substrate of a differentdie (substrates and die not visible in FIG. 5 ). In a second case, twoof portions 510-1, 2 and 3 are bonded to one substrate of one die; andthe other portion is bonded to a second substrate of a separate die. Ina third case, all three portions 510-1, 2 and 3 are bonded to a singleplate which is bonded to a single substrate of a single die.

The thickness of the piezoelectric plate portion 510-3 between its frontand back surfaces is the thinnest of all three portions 510-1, 2 and 3.The thickness of portion 510-1 is the thickest and that of portion 510-2is in between that of the thickness of other two portions. The thicknessof plate portion 510-3 is 730 nm. The thickness of plate portion 510-2is 744 nm. The thickness of plate portion 510-1 is 762 nm. Each of thesethree thicknesses may be plus or minus 10 nm.

The low-edge resonators XL1 and XL2 may be formed using or on thethickest plate portion, such as portion 510-1. Any high-edge resonatorwill be formed using or on the thinnest plate portion, such as portion510-3.

The matrix filter 500 includes eleven XBARs, such as the XBAR 520. Acavity (not visible) is formed in the substrates under each XBAR. EachXBAR is shown as a rectangle with vertical hatching and is identified bythe designator (XL1, X1A, . . . ) used in the schematic diagram of FIG.4 . The vertical hatching is representative of the direction of the IDTfingers of each XBAR but not to scale. Each XBAR has between 65 and 130IDT fingers, one of which is shown as finger 536. The IDT fingers arealuminum and 925 nm thick. The apertures AP (vertical direction as shownin FIG. 5 ) of the overlap of the interleaved fingers of the XBARs rangefrom 40 microns to 58 microns, and the lengths L (left-right directionas shown in FIG. 5 ) range from 500 to 1000 microns. In otherembodiments of XBAR matrix filters, the XBARs may be divided intosections to limit the length of the diaphragm within each XBAR. Thepitch p of the IDTs of each XBAR is between 7.5 and 8.6 microns and themark/pitch ratio of each XBAR is between 0.22 and 0.31.

The XBARs are connected to each other by conductors such as conductor530 that may also be formed on and connect between the substrates.Cross-hatched rectangles are metal-insulator-metal capacitors used asthe sub-filter coupling capacitors in this example of FIG. 4 , of whichonly capacitors 540 and 542 are identified. The identified capacitors540 and 542 are C3A and C3B, respectively, in the schematic diagram ofFIG. 4 . The other sub-filter coupling capacitors C1A-B and C2A-B forfilters 410-1 and 410-2, respectively, are connected similarly tocapacitors 540 and 542. The sub-filter coupling capacitors C1A-B, C2A-Band C3A-B are shown formed on and/or from the same plate portion 510-1,2 or 3 of the corresponding filter 410-1, 2 or 3, respectively, such asshown in the figure. However, the coupling capacitors may be formed onany of the plate portions 510-1, 2 or 3; or on separate portions of anyof the substrates. The capacitors C1A-B, C2A-B and C3A-B may be formedon the substrates 420-1, 2 or 3, respectively. The coupling capacitorsmay be separate from the substrates of portions 510, such as by beingdiscrete components or formed on a circuit card used to interconnect theresonators.

Connections from the filter 510 and circuitry external to the filter aremade by means of conductive pads indicated by shaded circles, such asconductive pad 550. The conductive pads for Filter Port 1 (FP1), FilterPort 2 (FP2), and ground (GND) are labeled. The three other conductivepads L11, L21 and L31 are connect to ground through inductor L1 (in FIG.4 ), which is located external to the filter 510. The conductor pads maybe connected using solder bumps or other connections to the pads. Theconductor pads may be formed on and/or be connect between thesubstrates.

As previously described, the sub-filters of a matrix filter havecontiguous passbands that span the passband of the matrix filter. Withina matrix filter, the center frequency of the passband of each sub-filteris different from the center frequency of any other sub-filter.Consequentially, the resonance frequencies of the XBARs in onesub-filter are different from the resonance frequencies of the XBARswithin any other sub-filter.

The resonance frequency of an XBAR is primarily determined by thethickness of the diaphragm or piezoelectric plate portion of thediaphragm within the XBAR. The resonance frequency has a smallerdependence on IDT pitch and mark or finger width. U.S. Pat. No.10,491,291 describes the use of a dielectric layer formed between theIDT fingers to adjust the resonance frequency of an XBAR. U.S. Pat. No.10,998,877 describes the use of the plate diaphragm portion thicknessesto adjust the resonance frequency of an XBAR.

FIG. 6 is a schematic cross-section view of the matrix filter 500 at asection plane D-D defined in FIG. 5 . The section plane D-D passesthrough one XBAR (X3A, X1A, X2A) from each of the three sub-filters inthe matrix filter 500. Each XBAR includes interleaved IDT fingers (ofwhich only IDT finger 630-1, 2 or 3 is identified) formed on arespective diaphragm spanning a respective cavity in a substrate 620-1,2 or 3 or a die (not visible). Substrates 620-1, 2 and 3 may be onesame, two different or three different substrates. Each substrate 620-1,2 or 3 may have other components of filter 500. Each diaphragm includesa different piezoelectric plate portion 510-1, 2 or 3 having a differentthickness between its front and back surfaces. Each of the front andback surfaces may be planar or flat across the entire surface of theplate or plate portion 510-1, 2 or 3. The back surface of each plateportion may be attached to one or more substrates of one or more die. Asin previous figures, the thickness of the piezoelectric plate portions510-1, 2 or 3 and the thickness, pitch, and finger width of the IDTs aregreatly exaggerated for visibility. Drawn to scale, the thickness of thepiezoelectric plate portions 10-1, 2 or 3 and the IDT fingers could beless than one-half percent of the thickness of the substrate 620-1, 2 or3 and each IDT would have 65 to 130 IDT fingers.

The three detail views illustrate the use of piezoelectric plate portionthickness to set the resonance frequencies of the XBARs within eachsub-filter. Consider first the detail view of an IDT finger of XBAR X1A(the middle view of the three detail views), which shows an IDT finger630-1 formed on a portion of the piezoelectric plate portion 510-1. TheIDT finger 630-1 is shown with a trapezoidal cross-section. Thetrapezoidal shape is exemplary and IDT fingers may have othercross-sectional shapes. The piezoelectric plate portion 510-1 hasthickness tp1 extending between its front surface 611-1 and its backsurface 612-1. Similarly, the right-hand detail shows piezoelectricplate portion 510-2 having thickness tp2 extending between its frontsurface 611-2 and its back surface 612-2. The left-hand detail showspiezoelectric plate portion 510-3 having thickness tp3 extending betweenits front surface 611-3 and its back surface 612-3. Each of thicknessestp1, tp2 and tp3 is different than any of the others.

In this example, XBAR X1A is an element of the sub-filter with thelowest passband frequency and XBAR X3A is an element of the sub-filterwith the highest passband frequency. In this case tp1>tp2>tp3≥0. Inother cases, two of the thickness tp1, tp2 and tp3 are the same but thethird thickness is greater than or less than those two thicknesses. Insome cases, there may be only two tp thicknesses, and in other casesthere may be more than three tp thicknesses.

Further in this example, XBARs X1B and X1C are also formed on portion510-1 with XBAR X1A as elements of the sub-filter 410-1 with the lowestpassband frequency. XBARs X3B and X3C are also formed on portion 510-3with XBAR X3A as elements of the sub-filter 410-3 with the highestpassband frequency. Finally, XBAR X2B and X2C are also formed on portion510-2 with XBAR X2A as elements of the sub-filter 410-2 with thepassband frequency between that of filters 410-1 and 410-3.

In a more general case where a matrix filter has n sub-filters, whichare numbered in order of increasing passband frequency, tp1>tp2> . .. >tpn, where tpi is the thickness of the piezoelectric plate portionextending between its front and a back surfaces of sub-filter i.

The low-edge resonators XL1 and XL2 may be formed using the thickest ofthe plate portions 510-1, 2 or 3. The low-edge resonators XL1 and XL2may have their resonance frequencies set by the thickness of the plateportion they are formed using. In addition, or independently, thelow-edge resonators XL1 and XL2 may have their resonance frequencies setby a thickness of a top layer dielectric. In this case, the spacebetween IDT fingers of XL1 and XL2 and adjacent IDT fingers (andoptionally the IDT finger) would be covered by a dielectric layer havinga thickness td1 and td2 to set the XL1 and XL2 resonance frequencies.The dielectric layers may be silicon dioxide, silicon nitride, aluminumoxide or some other dielectric material or combination of materials. Thedielectric layers may be the same or different materials.

An XBAR filter device typically includes a passivation dielectric layerapplied over the entire surface of the device, other than contact pads,to seal and passivate the conductor patterns and other elements of thedevice.

FIG. 7 is graph 700 of an estimated performance of a matrix filtersimilar to the matrix filter 500. The curve 710 is a plot of S21, theinput-output transfer function, of the filter determined by estimationof a physical model of the filter. The broken lines 720 mark the bandedges of 5G NR communication band n41. The matrix filter architectureextends the application of XBARs to lower frequency communications bandsthat are impractical using a conventional ladder filter architecture.

The concepts described above for FIGS. 5-7 may be applied to formfilters such as filter 500 but with different ranges and/or centerpoints of the pass band. For example, a filter similar to filter 500 maybe built with broken lines 720 that mark the band edges of 5G NRcommunication band n40 or n46.

Description of Methods

FIG. 8 is a simplified flow chart showing a process 800 for making anXBAR or a filter incorporating XBARs. The process 800 starts at 805 witha substrate and a plate of piezoelectric material and ends at 895 with acompleted XBAR or filter. It may be an example for forming any of theXBARs herein with any of the piezoelectric plate portions 610-1, 2 or 3of one or more piezoelectric plates one any one or more of substrates620-1, 2 or 3. The flow chart of FIG. 8 includes only major processsteps. Various conventional process steps (e.g. surface preparation,cleaning, inspection, baking, annealing, monitoring, testing, etc.) maybe performed before, between, after, and during the steps shown in FIG.8 .

The flow chart of FIG. 8 captures three variations of the process 800for making an XBAR which differ in when and how cavities are formed inthe substrate. The cavities may be formed at steps 810A, 810B, or 810C.Only one of these steps is performed in each of the three variations ofthe process 800.

The piezoelectric plate may be, for example, Z-cut lithium niobate orlithium tantalate with Euler angles 0, 0, 90°. The piezoelectric platemay be rotated Z-cut lithium niobate with Euler angles 0, β, 90°, whereβ is in the range from −15° to +5°. The piezoelectric plate may berotated Y-cut lithium niobate or lithium tantalate with Euler angles 0,β, 0, where β is in the range from 0 to 60°. The piezoelectric plate maybe some other material or crystallographic orientation. The substratemay preferably be silicon. The substrate may be some other material thatallows formation of deep cavities by etching or other processing.

In one variation of the process 800, one or more cavities are formed inthe substrate at 810A, before the piezoelectric plate is bonded to thesubstrate at 820. A separate cavity may be formed for each resonator ina filter device. The one or more cavities may be formed usingconventional photolithographic and etching techniques. Typically, thecavities formed at 810A will not penetrate through the substrate.

At 820, the piezoelectric plate is bonded to the substrate. Bonding at820 may be bonding any of the piezoelectric plates 610-1, 2 or 3 tosubstrate 620-1, 2 or 3. The piezoelectric plate and the substrate maybe bonded by a wafer bonding process. Typically, the mating surfaces ofthe substrate and the piezoelectric plate are highly polished. One ormore layers of intermediate materials, such as an oxide or metal, may beformed or deposited on the mating surface of one or both of thepiezoelectric plate and the substrate. One or both mating surfaces maybe activated using, for example, a plasma process. The mating surfacesmay then be pressed together with considerable force to establishmolecular bonds between the piezoelectric plate and the substrate orintermediate material layers.

A conductor pattern, including IDTs of each XBAR, is formed at 830 bydepositing and patterning two or more conductor levels on the front sideof the piezoelectric plate. The conductor levels typically include afirst conductor level that includes the IDT fingers, and a secondconductor level formed over the IDT busbars and other conductors exceptthe IDT fingers. In some devices, a third conductor levels may be formedon the contact pads. Each conductor level may be one or more layers of,for example, aluminum, an aluminum alloy, copper, a copper alloy, orsome other conductive metal. Optionally, one or more layers of othermaterials may be disposed below (i.e. between each conductor layer andthe piezoelectric plate) and/or on top of each conductor layer. Forexample, a thin film of titanium, chrome, or other metal may be used toimprove the adhesion between the first conductor level and thepiezoelectric plate. The second conductor level may be conductionenhancement layer of gold, aluminum, copper or other higher conductivitymetal may be formed over portions of the first conductor level (forexample the IDT bus bars and interconnections between the IDTs).

Each conductor level may be formed at 830 by depositing the appropriateconductor layers in sequence over the surface of the piezoelectricplate. The excess metal may then be removed by etching through patternedphotoresist. The conductor level can be etched, for example, by plasmaetching, reactive ion etching, wet chemical etching, and other etchingtechniques.

Alternatively, each conductor level may be formed at 830 using alift-off process. Photoresist may be deposited over the piezoelectricplate. and patterned to define the conductor level. The appropriateconductor layers may be deposited in sequence over the surface of thepiezoelectric plate. The photoresist may then be removed, which removesthe excess material, leaving the conductor level.

When a conductor level has multiple layers, the layers may be depositedand patterned separately. In particular, different patterning processes(i.e. etching or lift-off) may be used on different layers and/or levelsand different masks are required where two or more layers of the sameconductor level have different widths or shapes.

At 840, dielectric layers may be formed by depositing one or more layersof dielectric material on the front side of the piezoelectric plate. Aspreviously described, the dielectric layers may include a differentdielectric thickness over the IDT fingers of the XBARs within eachsub-filter. Each dielectric layer may be deposited using a conventionaldeposition technique such as sputtering, evaporation, or chemical vapordeposition. Each dielectric layer may be deposited over the entiresurface of the piezoelectric plate, including on top of the conductorpattern. Alternatively, one or more lithography processes (usingphotomasks) may be used to limit the deposition of the dielectric layersto selected areas of the piezoelectric plate, such as only between theinterleaved fingers of the IDTs. Masks may also be used to allowdeposition of different thicknesses of dielectric materials on differentportions of the piezoelectric plate.

The matrix filter shown in FIG. 5 and FIG. 6 includesmetal-insulator-metal (MIM) capacitors. A MIM capacitor consists of afirst metal level and a second metal level separated by a dielectriclayer. When a matrix filter includes MIM capacitor, the steps of formingthe conductor patterns at 830 and forming the dielectric layers at 840must overlap. At least one dielectric layer has to be formed at 840after a first metal level is formed at 830 and before a final metallevel is formed at 830. The MIM capacitors may be formed beside or onany of the piezoelectric plate portions 610-1, 2 or 3; or substrates620-1, 2 or 3.

In a second variation of the process 800, one or more cavities areformed in the back side of the substrate at 810B. A separate cavity maybe formed for each resonator in a filter device. The one or morecavities may be formed using an anisotropic or orientation-dependent dryor wet etch to open holes through the back side of the substrate to thepiezoelectric plate. In this case, the resulting resonator devices willhave a cross-section as shown in FIG. 1 .

In the second variation of the process 800, a back-side dielectric layermay be formed at 850. In the case where the cavities are formed at 810Bas holes through the substrate, the back-side dielectric layer may bedeposited through the cavities using a conventional deposition techniquesuch as sputtering, evaporation, or chemical vapor deposition.

In a third variation of the process 800, one or more cavities in theform of recesses in the substrate may be formed at 810C by etching thesubstrate using an etchant introduced through openings in thepiezoelectric plate. A separate cavity may be formed for each resonatorin a filter device.

In all variations of the process 800, the filter device is completed at860. Actions that may occur at 860 include depositing anencapsulation/passivation layer such as SiO₂ or Si₃O₄ over all or aportion of the device; forming bonding pads or solder bumps or othermeans for making connection between the device and external circuitry;excising individual devices from a wafer containing multiple devices;other packaging steps; and testing. Another action that may occur at 860is to tune the resonant frequencies of the resonators within the deviceby adding or removing metal or dielectric material from the front sideof the device. After the filter device is completed, the process ends at895.

The descriptions herein such as for FIGS. 4-8 provide improved XBARradio frequency filter configurations by using XBAR sub-filters withdifferent thicknesses of the piezoelectric plate portions connected inparallel between two ports. The sub-filters have a piezoelectric platewith a back surface attached to the different substrates and portions ofthe plate forming diaphragms spanning cavities in the substrates.Interleaved fingers of IDTs are on the diaphragms. The differentthickness portions of the piezoelectric plates may be on differentsubstrates of different die connected in parallel between two ports.

These configurations form a distributed (matrix) XBAR filter that allowsfor the reduction of required resonator static capacitance C0, andtherefore a reduction in required die area. These configurations arealso scalable to arbitrary order and can readily be made reconfigurablewith the use of RF switches. By incorporating the configurations'multi-die approach, similar to a ‘split ladder’ topology, additionalfreedom in the design of the distributed filter is achieved.

Without these configurations, constraining all resonators of multiplesub-filters to a single die requires frequency separation of resonatorsto be achieved by varying top layer oxide and/or electrode dimensions.Instead, the multi-die configurations introduce the membrane thicknessas an additional degree of freedom that may be applied by sub-filterresonator groups.

CLOSING COMMENTS

Throughout this description, the embodiments and examples shown shouldbe considered as exemplars, rather than limitations on the apparatus andprocedures disclosed or claimed. Although many of the examples presentedherein involve specific combinations of method acts or system elements,it should be understood that those acts and those elements may becombined in other ways to accomplish the same objectives. With regard toflowcharts, additional and fewer steps may be taken, and the steps asshown may be combined or further refined to achieve the methodsdescribed herein. Acts, elements and features discussed only inconnection with one embodiment are not intended to be excluded from asimilar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set”of items may include one or more of such items. As used herein, whetherin the written description or the claims, the terms “comprising”,“including”, “carrying”, “having”, “containing”, “involving”, and thelike are to be understood to be open-ended, i.e., to mean including butnot limited to. Only the transitional phrases “consisting of” and“consisting essentially of”, respectively, are closed or semi-closedtransitional phrases with respect to claims. Use of ordinal terms suchas “first”, “second”, “third”, etc., in the claims to modify a claimelement does not by itself connote any priority, precedence, or order ofone claim element over another or the temporal order in which acts of amethod are performed, but are used merely as labels to distinguish oneclaim element having a certain name from another element having a samename (but for use of the ordinal term) to distinguish the claimelements. As used herein, “and/or” means that the listed items arealternatives, but the alternatives also include any combination of thelisted items.

It is claimed:
 1. A method of forming a radio frequency filter,comprising: connecting a first sub-filter and a second sub-filter inparallel between a first port and a second port, wherein connecting eachof the first and second sub-filters comprises: bonding to a substrate aback surface of a piezoelectric plate having front and back surfaces,portions of the piezoelectric plate forming a plurality of diaphragmsspanning respective cavities in the substrate; and forming a conductorpattern on the front surface, the conductor pattern including aplurality of interdigital transducers (IDTs) of a respective pluralityof resonators, interleaved fingers of each IDT disposed on a respectivediaphragm of the plurality of diaphragms, wherein a thickness of theportion of the piezoelectric plate of the first sub-filter is differentfrom a thickness of the portion of the piezoelectric plate of the secondsub-filter, and wherein the substrate of the first sub-filter isdifferent than the substrate of the second sub-filter.
 2. The method ofclaim 1, wherein the piezoelectric plate of the first sub-filter is thesame as the piezoelectric plate of the second sub-filter.
 3. The methodof claim 1, wherein the piezoelectric plate of the first sub-filter isdifferent than the piezoelectric plate of the second sub-filter.
 4. Themethod of claim 1, wherein: the first and second portions and the firstand second IDTs are configured such that radio frequency signals appliedto the first and second IDTs excite primary shear acoustic modes in thefirst and second portions of the piezoelectric plates forming theplurality of diaphragms spanning the respective cavities in thedifferent substrates; and the thicknesses of the first and secondportions are selected to tune the primary shear acoustic modes in thefirst and second portions.
 5. The method of claim 1, further comprising:connecting a third sub-filter in parallel between the first port and thesecond port, wherein connecting the third sub-filter comprises: bondingto a substrate a back surface of a piezoelectric plate having front andback surfaces, portions of the piezoelectric plate forming a pluralityof diaphragms spanning respective cavities in the substrate; and forminga conductor pattern on the front surface, the conductor patternincluding a plurality of interdigital transducers (IDTs) of a respectiveplurality of resonators, interleaved fingers of each IDT disposed on arespective diaphragm of the plurality of diaphragms, wherein a thicknessof the piezoelectric plate portion of the third sub-filter is differentfrom a thickness of the piezoelectric plate portion of the first andportion of the second sub-filters.
 6. The method of claim 5, wherein thethickness of the piezoelectric plate portion of the first sub-filter isa thickness of between 720 nm and 740 nm extending between a frontsurface and a back surface of the piezoelectric plate portion of thefirst sub-filter; wherein the thickness of the piezoelectric plateportion of the second sub-filter is a thickness of between 752 nm and772 nm extending between a front surface and a back surface of thepiezoelectric plate portion of the second sub-filter; and wherein thethickness of the piezoelectric plate portion of the third sub-filter isa thickness of between 734 nm and 754 nm extending between a frontsurface and a back surface of the piezoelectric plate portion of thethird sub-filter.
 7. The method of claim 5, wherein connecting each ofthe three sub-filters further comprises: connecting three resonators inseries between the first port and the second port; and connecting twocoupling capacitors between ground and a respective node between two ofthe resonators of the sub-filter.
 8. The method of claim 7, furthercomprising: connecting a first low-edge resonator, from the plurality ofresonators, between the first port and ground; connecting a secondlow-edge resonator, from the plurality of resonators, between the secondport and ground; wherein respective resonance frequencies of the firstand second low-edge resonators are adjacent to a lower edge of apassband of the filter.
 9. The method of claim 8, wherein: two of theresonators of each sub-filter are symmetrical in response; the low-edgeresonators have the same response; each of the coupling capacitors is ametal-insulator-metal capacitor; and the sub-filters and the low-edgeresonators form a matrix filter having a contiguous passband formed bypassbands of the sub-filters; and a center frequency of a passband ofeach sub-filter is different from a center frequency of any othersub-filter.
 10. The method of claim 5, wherein: the thickness of thepiezoelectric plate portion of the first sub-filter is thinner than thethickness of the piezoelectric plate portion of the second sub-filter;and the thickness of the piezoelectric plate portion of the secondsub-filter is thinner than the thickness of the piezoelectric plateportion of the third sub-filter.
 11. A method of forming a radiofrequency filter, comprising: connecting a first sub-filter and a secondsub-filter in parallel between a first port and a second port, whereinconnecting the first sub-filter comprises: bonding to a first substratea first back surface of a first piezoelectric plate having first frontand back surfaces, first portions of the first piezoelectric plateforming a first plurality of diaphragms spanning respective firstcavities in the first substrate; and forming a first conductor patternon the first front surface, the first conductor pattern including afirst plurality of interdigital transducers (IDTs) of a first respectiveplurality of resonators, first interleaved fingers of each IDT disposedon a first respective diaphragm of the first plurality of diaphragms,wherein connecting the second sub-filters comprises: bonding to a secondsubstrate a second back surface of a second piezoelectric plate havingsecond front and back surfaces, second portions of the secondpiezoelectric plate forming a second plurality of diaphragms spanningrespective second cavities in the second substrate; and forming a secondconductor pattern on the second front surface, the second conductorpattern including a second plurality of interdigital transducers (IDTs)of a second respective plurality of resonators, second interleavedfingers of each IDT disposed on a second respective diaphragm of thesecond plurality of diaphragms, wherein a thickness of the firstpiezoelectric plate first portions are thicker than a thickness of thesecond piezoelectric plate second portions, and wherein the firstsubstrates is different than the second substrate.
 12. The method ofclaim 11, wherein the first piezoelectric plate is the same as thesecond piezoelectric plate.
 13. The method of claim 11, wherein thefirst piezoelectric plate is different than the second piezoelectricplate.
 14. The method of claim 11, wherein: the first and secondportions and the first and second IDTs are configured such that radiofrequency signals applied to the first and second IDTs excite primaryshear acoustic modes in the first and second portions of the first andsecond piezoelectric plate portions forming the first and secondplurality of diaphragms spanning the first and second respectivecavities in the first and second different substrates; and thethicknesses of the first and second portions are selected to tune theprimary shear acoustic modes in the first and second portions.
 15. Themethod of claim 11, wherein connecting each of the first and secondsub-filters further comprises: connecting three resonators connected inseries between the first port and the second port; connecting each oftwo coupling capacitors between ground and a respective node between twoof the resonators of the sub-filter; and further comprising: connectinga first low-edge resonator, from the plurality of resonators, betweenthe first port and ground; connecting a second low-edge resonator, fromthe plurality of resonators, between the second port and ground; whereinrespective resonance frequencies of the first and second low-edgeresonators are adjacent to a lower edge of a passband of the filter. 16.A method of forming radio frequency filter, comprising: connecting afirst sub-filter, a second and a third sub-filter in parallel between afirst port and a second port, wherein connecting each of the first,second and third sub-filters comprises: bonding to a substrate a backsurface of a piezoelectric plate having front and back surfaces,portions of the piezoelectric plate forming a plurality of diaphragmsspanning respective cavities in the substrate; and forming a conductorpattern on the front surface, the conductor pattern including aplurality of interdigital transducers (IDTs) of a respective pluralityof resonators, interleaved fingers of each IDT disposed on a respectivediaphragm of the plurality of diaphragms, wherein a thickness of thepiezoelectric plate portion of the first sub-filter is different from athickness of the piezoelectric plate portion of the second and thirdsub-filter, a thickness of the piezoelectric plate portion of the secondsub-filter is different from a thickness of the piezoelectric plateportion of the third sub-filter, wherein the substrate of the firstsub-filter different than the substrate of the second sub-filter and thethird sub-filter, and the substrate of the second sub-filter isdifferent than the substrate of the third sub-filter.
 17. The method ofclaim 16, wherein the piezoelectric plate of the first sub-filterdifferent than the piezoelectric plate of the second sub-filter and thethird sub-filter, and wherein the piezoelectric plate of the secondsub-filter is different than the piezoelectric plate of the thirdsub-filter.
 18. The method of claim 16, wherein: the piezoelectric plateportions and the IDTs of the first, second and third sub-filter areconfigured such that radio frequency signals applied to the IDTs exciteprimary shear acoustic modes in the portions of the piezoelectric plateportions forming the plurality of diaphragms spanning the respectivecavities in the different substrates; and the thicknesses of thepiezoelectric plate portions of the first, second and third sub-filtersare selected to tune the primary shear acoustic modes in the portions ofthe piezoelectric plates.